1. Field of the Invention
The present invention relates to an electrophotographic imaging apparatus such as a copying machine, a laser beam printer, and a facsimile machine, and a process cartridge and toner that are used in the electrophotographic imaging apparatus.
2. Description of the Related Art
Conventionally, in the field of electrophotographic imaging apparatuses such as copying machines, laser beam printers, and facsimile machines, an imaging technique of forming a latent image by charging a surface of a photoconductor corresponding to an image carrier is known.
Currently, a technique is being developed for decreasing the particle diameter and increasing the roundness of toner used in an imaging apparatus in order to improve the output image quality. In such case, there is a limit to decreasing the particle diameter and increasing the roundness of the toner produced by a conventional pulverization method. Thereby, toner produced by a polymerization method is starting to be used to further decrease the particle diameter and increase the roundness of toner. The polymerization method includes suspension polymerization, emulsification polymerization, and dispersion polymerization, for example, which enable production of round toner particles.
It is known that toner with high roundness has inferior cleaning characteristics. Particularly, toner produced by the polymerization method may have roundness close to a sphere (e.g., average roundness of 0.98 or more), and thereby, it is difficult to clean the polymerized toner by means of a conventional cleaning method for pulverized toner using a cleaning blade. Specifically, the toner particles of the polymerized toner may not be stuck to the edge of the cleaning blade, and may instead slide across the image carrier (photoconductor) surface. Thereby, the toner particles are prone to pass around the cleaning blade, causing a fault in the cleaning process. It is noted that the method for cleaning the toner particles is not limited to the blade cleaning method, and other methods such as brush cleaning, magnetic brush cleaning, and electrostatic brush cleaning may be used as well. From the aspect of cleaning performance and cost, a combination of the blade cleaning method and the brush cleaning method is generally used. A number of techniques have been proposed in the prior art for improving the toner cleaning performance for very round toner particles.
For example, Japanese Patent Laid-Open Publication No. 5-107990 discloses a cleaning apparatus implementing a pre-cleaning charge unit for applying an electric charge with the same polarity as that of the toner to an upstream side of a conductive brush of an image carrier, a bias applying member attached to the conductive brush and including at least a bias with an opposite polarity to that of the charge of the pre-cleaning unit, and, if desired, a pre-cleaning exposure unit that is positioned at the same region as that of the pre-cleaning charge unit or positioned downstream of the pre-cleaning charge unit and upstream of the conductive brush, wherein a charge with the same polarity as that of the toner is applied to the image carrier by the pre-cleaning charge unit to neutralize the charge of carriers residing in small amounts on the surface of the image carrier and to reduce the adhesiveness of the carriers to the image carrier. In this way, carriers on the image carrier may be removed, and the carriers may be prevented from reaching a blade region so that the image carrier surface at the blade region may be protected from damage. However, since the charge of the toner on the image carrier is increased in this example, electrostatic attraction between the toner and the image carrier (photoconductor) is increased, and blade cleaning becomes difficult for very round toner particles.
Also, Japanese Patent Laid-Open Publication No. 8-248849 discloses a cleaning apparatus implementing a direct current power source and an indirect current power source that apply to a cleaning brush a direct current and an indirect current that are superimposed on each other, the direct current power source and the indirect current power source being positioned upstream of the cleaning brush with respect to a rotational direction of a photoconductor and downstream of a transfer unit with respect to the rotational direction of the photoconductor. In this way, the surface of the photoconductor may be arranged to have the same polarity as that of a remaining developing agent so that the electrostatic attraction of the developing agent to the photoconductor may be weakened to thereby improve the cleaning performance. However, according to the present related art example, the electric potential of the photoconductor surface is reversed so that the service life of the photoconductor may possibly be influenced.
Also, Japanese Patent Laid-Open Publication No. 2000-267536 discloses an imaging apparatus implementing an image carrier cleaning blade of which a blade edge is coated with a powdery mixture material. According to this example, a suitable toner dam may be formed at a nip of the image carrier and the blade edge from the initial stage of using the imaging apparatus, and spherical toner particles may be prevented from slipping past the blade even when a large amount of toner particles are applied to the blade edge. However, it is difficult to evenly apply the toner powdery mixture material on the surface of the blade, and problems also arise with respect to pressure resistance.